1. Vertical intensity of the earth's magnetic field, epoch 1975.0 Yale Public content 1975. 2018-04-10T17:53:11Z. 2018-04-10T17:53:11Z. United States. Defense Mapping Agency. Hydrographic/Topographic Center. The Center : [DMA, Combat Support Center, DOA, distributor]. English. cartographic image. Public.